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Register a new userMixing and nucleosynthesis in rotating TP-AGB stars
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N. Langer
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We present the first evolutionary models of intermediate mass stars up to their thermal pulses which include effects of rotation on the stellar structure as well as rotationally induced mixing of chemical species and angular momentum. We find a significant angular momentum transport from the core to the hydrogen-rich envelope and obtain a white dwarf rotation rate comparable to current observational upper limits of 50 km/s. Large angular momentum gradients at the bottom of the convective envelope and the tip of the pulse driven convective shell are shown to produce marked chemical mixing between the proton-rich and the 12C-rich layers during the so called third dredge-up. This leads to a subsequent production of 13C which is followed by neutron production through 13C(alpha,n) in radiative layers in between thermal pulses. Although uncertainties in the efficiency of rotational mixing processes persist, we conclude that rotation is capable of producing a 13C-rich layer as required for the occurrence of the s-process in TP-AGB stars.
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Based on evolutionary computations of 90 stellar models, we have analysed the impact of initial composition and core overshooting on the post-He-burning evolution and the associated nucleosynthesis of Super-AGB stars, pointing particular attention on the C-burning phase. Moreover the possible link between the transition masses $M_{up}$, $M_{N}$ and $M_{mas}$ (defined as the critical initial mass above which C-burning ignites, the minimum initial mass for an electron-capture supernova and the minimum initial mass for the completion of all the nuclear burning phases respectively) and the properties of the core during the core He-burning phase is also briefly discussed.
Aims. We investigate the s-process during the AGB phase of stellar models whose cores are enforced to rotate at rates consistent with asteroseismology observations of their progenitors and successors. Methods. We calculated new 2M$_{odot}$, Z=0.01 models, rotating at 0, 125, and 250 km/s at the start of main sequence. An artificial, additional viscosity was added to enhance the transport of angular momentum in order to reduce the core rotation rates to be in agreement with asteroseismology observations. We compared rotation rates of our models with observed rotation rates during the MS up to the end of core He burning, and the white dwarf phase. Results. We present nucleosynthesis calculations for these rotating AGB models that were enforced to match the asteroseismic constraints on rotation rates of MS, RGB, He-burning, and WD stars. In particular, we calculated one model that matches the upper limit of observed rotation rates of core He-burning stars and we also included a model that rotates one order of magnitude faster than the upper limit of the observations. The s-process production in both of these models is comparable to that of non-rotating models. Conclusions. Slowing down the core rotation rate in stars to match the above mentioned asteroseismic constraints reduces the rotationally induced mixing processes to the point that they have no effect on the s-process nucleosynthesis. This result is independent of the initial rotation rate of the stellar evolution model. However, there are uncertainties remaining in the treatment of rotation in stellar evolution, which need to be reduced in order to confirm our conclusions, including the physical nature of our approach to reduce the core rotation rates of our models, and magnetic processes.
We examine nucleosynthesis in the electron capture supernovae of progenitor AGB stars with an O-Ne-Mg core (with the initial stellar mass of 8.8 M_odot). Thermodynamic trajectories for the first 810 ms after core bounce are taken from a recent state-of-the-art hydrodynamic simulation. The presented nucleosynthesis results are characterized by a number of distinct features that are not shared with those of other supernovae from the collapse of stars with iron core (with initial stellar masses of more than 10 M_odot). First is the small amount of 56Ni (= 0.002-0.004 M_odot) in the ejecta, which can be an explanation for observed properties of faint supernovae such as SNe 2008S and 1997D. In addition, the large Ni/Fe ratio is in reasonable agreement with the spectroscopic result of the Crab nebula (the relic of SN 1054). Second is the large production of 64Zn, 70Ge, light p-nuclei (74Se, 78Kr, 84Sr, and 92Mo), and in particular, 90Zr, which originates from the low Y_e (= 0.46-0.49, the number of electrons per nucleon) ejecta. We find, however, that only a 1-2% increase of the minimum Y_e moderates the overproduction of 90Zr. In contrast, the production of 64Zn is fairly robust against a small variation of Y_e. This provides the upper limit of the occurrence of this type of events to be about 30% of all core-collapse supernovae.
We review the current state of modeling convective mixing in AGB stars. The focus is on results obtained through multi-dimensional hydrodynamic simulations of AGB convection, both in the envelope and the unstable He-shell. Using two different codes and a wide range of resolutions and modeling assumptions we find that mixing across convective boundaries is significant for He-shell flash convection. We present a preliminary quantitative analysis of this convectively induced extra mixing, based on a sub-set of our simulations. Other non-standard mixing will be discussed briefly.
The spectro-photometric properties of galaxies in galaxy formation models are obtained by combining the predicted history of star formation and mass accretion with the physics of stellar evolution through stellar population models. In the recent literature, significant differences have emerged regarding the implementation of the Thermally-Pulsing Asymptotic Giant Branch phase of stellar evolution. The emission in the TP-AGB phase dominates the bolometric and near-IR spectrum of intermediate-age (~1 Gyr) stellar populations, hence it is crucial for the correct modeling of the galaxy luminosities and colours. In this paper for the first time, we incorporate a full prescription of the TP-AGB phase in a semi-analytic model of galaxy formation. We find that the inclusion of the TP-AGB in the model spectra dramatically alters the predicted colour-magnitude relation and its evolution with redshift. When the TP-AGB phase is active, the rest-frame V-K galaxy colours are redder by almost 2 magnitudes in the redshift range z~2-3 and by 1 magnitude at z~1. Very red colours are produced in disk galaxies, so that the V-K colour distributions of disk and spheroids are virtually undistinguishable at low redshifts. We also find that the galaxy K-band emission is more than 1 magnitude higher in the range z~1-3. This may alleviate the difficulties met by the hierarchical clustering scenario in predicting the red galaxy population at high redshifts. The comparison between simulations and observations have to be revisited in the light of our results.
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